To make a smooth migration from the third generation (3G) mobile communication to the fourth generation (4G) mobile communication, a long term evolution (LTE: super 3G) has been developed as a standard for high-speed mobile communication. Although the LTE is regarded as an evolution of the international mobile telecommunications (IMT)-2000, a wireless interface and a radio access controller (RAC) architecture of the 3G system have been reexamined fundamentally to make a smooth migration to the 4G. As a result, with delay time compression and improvement in communication speed, the LTE achieves maximum data communication speed of 300 megabits per second for downloading, and of 50 megabits per second for uploading.
Mobile communication systems according to the LTE have undergone some changes in nodes from the current 3G systems. Specifically, because a plurality of wireless base stations is centrally managed by an evolved packet core (EPC), radio network controllers (RNCs) provided in conventional 3G systems are omitted and most functions of the RNCs are provided in the wireless base stations.
In communication between wireless base stations according to the LTE, a message in an S1 protocol is used when the communication is performed through the EPC, while a message in an X2 protocol is used when the communication is performed not through the EPC, as specified in the recommendations of the third generation partnership project (3GPP). To use such messages in the X2 protocol and the S1 protocol, it is necessary for each wireless base station to establish setting links to another wireless base station to communicate with and to the EPC, respectively.
In general, such an inter-station link is set or removed when a wireless base station is set up, or when the number of neighboring wireless base stations is increased or decreased. Desirably, inter-station links are set for all of the neighboring base stations. Under existing circumstances, however, when inter-station links are set for many neighboring wireless base stations, the processes for setting the inter-station links are increased, thus causing time delay in restarting mobile communication interrupted during the setting. Further, when many inter-station links are set, resources necessary for setting the inter-base links run out because they are limited. For these reasons, attempts have been made to reduce the number of neighboring wireless base stations for which inter-station links are set as much as possible.
Considering the existing circumstances for setting the inter-station links, the recommendations of the 3GPP define a process for switching a wireless link between a mobile device and a wireless base station according to movement of the mobile device, when the mobile device moves from a wireless base station to another wireless base station (the process is referred to as “handover”). Specifically, the 3GPP defines: X2 handover using an inter-station link between wireless base stations; and S1 handover using an inter-station link between a wireless base station and an EPC, either of which is used according to the movement of a mobile device from a wireless base station to another wireless base station.
Referring to FIG. 16, the following describes operations of handover in mobile communication using the LTE. As shown in FIG. 16, in the mobile communication system using the LTE, a plurality of wireless base stations (A, B, X, Y, and C) is directly connected to an EPC, and an inter-station link is set between each of the wireless base stations and the EPC. The wireless base station X has an inter-station link to the wireless base station A, while having no inter-station link to the other wireless base stations B, Y, and C. In this situation, for example, when a mobile device being in wireless communication with the wireless base station X moves into a control region of the wireless base station A, X2 handover is performed because the wireless base station A has an inter-station link to the wireless base station X. On the contrary, when a mobile device being in wireless communication with the wireless base station X moves into a control region of the wireless base station C, X2 handover cannot be performed and S1 handover is performed because the wireless base station C has no inter-station link to the wireless base station X.
The PCT International Publication WO99/34627 Pamphlet discloses the following technology. Assume that a mobile device being in wireless communication with the wireless base station X moves into a region where no wireless base station exists as a recipient of handover. In this case, by reducing the number of handover requests regularly made by the mobile device, the voice interruption caused by handover operations is reduced. In this way, the communication quality is improved between the mobile device and a wireless base station.
When handover is made from a wireless base station being in wireless communication with many mobile devices to another wireless base station having no inter-station link to the wireless base station, S1 handover is performed for every handover. This poses a problem of increasing the processing load on the EPC.
Referring to FIG. 17, the following describes the problem regarding the load on the EPC in detail. As shown in FIG. 17, the wireless base stations X and C each have an inter-station link to the EPC, while having no inter-station link therebetween. In this situation, when many mobile devices being in wireless communication with the wireless base station X move into a control region of the wireless base station C, the wireless base stations X and C cannot perform X2 handover because no inter-station link has been set therebetween. Thus, the wireless base stations X and C perform S1 handover every time such a mobile device moves in. This increases the processes for performing S1 handover in the EPC, thus increasing the processing load thereon.
Particularly in the LTE, it is assumed that one EPC controls many wireless base stations. When a plurality of wireless base stations makes many requests for S1 handover at one time, the EPC has a further increased processing load. As a result, the handover may fail, degrading the services to be provided to existing callers.